♪♪ Attenborough: I'm on the south coast of England in what is known as the Jurassic Coast.

150 million years ago, the land was ruled by dinosaurs.

But the oceans were dominated by a mysterious sea monster known as a pliosaur.

Its remains are very rare, but some have just been discovered in the cliffs behind me.

This could be one of the largest, best-preserved pliosaur skulls ever found.

And now working with scientists, we can uncover more about this prehistoric giant than ever before.

What an extraordinary, terrifying thing.

Could this be the greatest Jurassic predator... ...that ever lived?

♪♪ [ Seabirds calling ] ♪♪ Attenborough: Britain has a wealth of fossil science, but perhaps none are quite so famous as this, the Jurassic Coast.

I've been collecting fossils since I was a boy, and I haven't yet got tired of it.

These days, you're supposed to wear glasses for safety.

And if you would know a locality, you would begin to recognize the sort of block that might contain a fossil.

And this rock was split earlier to make it easier to show you.

And if I can hit it quite hard about there... What about that?

Is anything more beautiful than that?

Fantastic.

Finding even the smallest fossils is a thrill, but discovering something much larger can be an unforgettable moment.

And that is exactly what happened to a fossil enthusiast who was walking along a beach near Kimmeridge Bay in Southwest England one morning.

Man: I just found something quite extraordinary.

It's the jaw of a massive pliosaur.

It's enormous.

It's a massive pliosaur; it's the best fossil I've ever found.

It must have just come out of the cliffs up there somewhere.

Attenborough: Renowned fossil expert Dr. Steve Etches is called in, and he knows immediately that it is hugely significant.

Etches: Look, look at those teeth.

That's a tooth there, look.

Let's have a look at the front.

You can walk along here for hundred of times and not find anything and then occasionally, you'll hit the jackpot.

Attenborough: And this certainly is the jackpot.

It's the snout of an enormous pliosaur skull.

And Steve suspects that the rest of the head might still be imbedded in the cliff above.

But he needs to work fast before the whole thing tumbles into the sea and is lost forever.

So he quickly gathers a team of experts, including fellow fossil hunter Chris Moore.

Moore: It's day one of the dig.

Steve and team and diggers are up on the cliffs, and they're putting a net down to stop any loose rock rolling down and hitting us.

And then we'll go down and expose the skull.

[ Drill pounding ] Attenborough: Steve and Chris have worked together for decades, but this is the biggest challenge they have ever faced.

The skull is nearly 40 feet up from the base of the cliff... ...and lying upside down in a horizontal position.

Its full length is not yet known, but Steve believes it could be over 6 feet long.

But how did the fossil end up here?

Well, these rocks were once mud on the seafloor... ...in which the remains of prehistoric marine creatures were buried.

Over millions of years, the continents shifted... the seas receded... and today, as these cliffs erode, fossilized skeletons are revealed.

Etches: The jaw is there and the idea is to actually cut this down vertically and then we form a platform where the fossil is.

♪♪ Attenborough: I've known Chris and Steve for many years, and I can't wait to join them and find out how they're getting on.

♪♪ Moore: Here we've got a live feed, actually digging halfway up the cliff face.

What's happened so far -- the tip of the snout has already come out and been recovered, so the rest of it is going into the cliff.

Attenborough: What exactly are they doing there?

Moore: At the moment, they are using tinfoil to try and protect the exposed piece of jaw... Attenborough: In case something falls on it and breaks it.

Moore: Yeah, yeah, and then they're also using Superglue to consolidate the crumbly teeth and parts that have been weathered over millenniums.

Attenborough: And they are hanging by the ropes there.

Moore: Yes.

Pretty much in the center of the cliff.

Attenborough: Dear me.

Pretty dangerous stuff, this.

Moore: Yeah, yeah, it really is.

After you've worked there for a few hours, it becomes a bit more natural and you concentrate on the actual digging.

Attenborough: You must be pretty sure that there is something there to put this amount of effort into it, really.

Moore: Oh, positive there is something there.

It's beautiful.

It's a beautiful specimen.

-Attenborough: Is it?

-Moore: Yeah, amazing.

Attenborough: But what can this spectacular find reveal about the lives of these mysterious sea creatures and the world they inhabited?

In the late Jurassic, Europe was an archipelago of islands, much closer to the equator than it is today.

Its seas were warm, shallow and teeming with life.

And in these waters underneath my feet lurked the ultimate marine predator, the pliosaur.

♪♪ ♪♪ Unlike dinosaurs that lived on land, these colossal marine reptiles spent their whole lives in the ocean.

Thought to have been over 30 feet in length, similar to a double-decker bus.

They had long, broad flippers, short, strong necks, huge heads... and enormous jaws.

But there's still a lot we don't know about these great sea monsters, which is why this new discovery is so important.

Steve believes that the entire pliosaur may still be inside the cliff, but it's the skull on which he is concentrating.

A skull can reveal more about an animal than any other part of its skeleton.

♪♪ Finding a complete specimen is rare, but it can tell us a great deal about how the animal lived.

♪♪ [ Drill pounding ] Moore: It's quite hot and thirsty work.

Attenborough: Steve and Chris think they will have about three weeks to dig the fossil out before the storms of late summer come.

Etches: This is a learning curve.

I thought, stupidly, that it wouldn't be quite as hard as this.

We're probably over a meter in.

There's the skull there just behind me.

It's up to us now to get it out without any damage.

Attenborough: If the skull is successfully extracted, it will be taken to the Etches Collection Museum in Kimmeridge, which was founded by Steve in 2016 and now contains nearly 3,000 fossils from the Jurassic period.

This was where the pliosaur snout was brought to be assessed.

And I've come here to learn what Steve has found out so far.

What an extraordinary, terrifying thing.

Huge teeth.

Etches: They are.

They're massive.

These big, fang-like teeth that come up.

Attenborough: And they interlock.

Etches: Yes, they do.

Attenborough: So how far do these teeth extend?

Etches: They come right up here.

Attenborough: Is that characteristic of this?

Etches: Yes, typical pliosaur.

Trihedral teeth, they're sort of...

They got two sharp, cutting edges and then a flat face on the inside.

Attenborough: And they must have used that to rip apart ichthyosaurs.

Etches: Ichthyosaurs, plesiosaurus as well.

We've got evidence in the museum because they've got sub-triangular teeth, when they bite in the bone, they leave a very distinctive tooth hole.

Attenborough: Steve's collection contains a limb bone of a plesiosaur, another large marine reptile, which has bite marks on it that exactly match the size and shape of pliosaur teeth.

What a godsend it is that the teeth are triangular so that you are able to then identify prey... -Etches: Oh, yeah.

-Attenborough: ...just as much as you can identify the predator.

Etches: That's right.

Attenborough: That's pretty unusual to find teeth in position.

-Etches: It is.

This is the first pliosaur I've ever seen with actually closed jaws.

Attenborough: It is extraordinary.

It is quite extraordinary.

And there is another unusual feature on this snout.

Etches: So let me show you the other side.

I'll spin it 'round.

Attenborough: Oh, gosh, there's a lot of stuff here.

Etches: Oh, yeah.

Attenborough: And these, these holes here... Etches: Why I think these are sensory pits, they're all on the snout.

If you look at them, they go... See, they strike back in at an angle, you know?

They're sensory pits for picking up his prey, I'm pretty sure.

Pretty impressive beast.

Attenborough: Astonishing.

But exactly how did these sensory pits help our pliosaur to hunt?

These days, we have equipment that can help us answer such a question.

We take the snout to the University of Southampton, which has some of the most powerful CT scanners in the UK.

Once the snout has been scanned, paleobiologist Dr. Neil Gostling looks for clues about the sensory abilities of our pliosaur.

So what does this scan tell us?

Gostling: I've taken the CT data, and we can rack our way through this stack of images and we can see all of those internal structures which otherwise wouldn't have been revealed to us.

And there were some things, which are, I think, quite exciting.

We've got these little structures in red and I had to do these each one, slice by slice, dot by dot, because there's lots of imperfections and holes in it, but these are continuous.

And I think these are blood vessels, little branching blood vessels.

Attenborough: Oh, come on.

You're really telling me this block of stone can show blood vessels?

Gostling: Yes, and what I think we've got here are actually branches of the trigeminal nerve and these are the sensory nerves in your face that allow you to feel fingers dancing over your cheeks and what have you.

But if you're in water and you've got these sensory pits, you can detect changes in pressure.

That's going to give you an advantage in what might be murky water where your eyes aren't working so well, so you can always be sure that you're going to catch your next meal.

Attenborough: Fantastic.

Knowing how these senses worked, we can start to build up a picture of how our pliosaur hunted.

Its prey... ichthyosaurs -- reptiles much the same shape as a modern dolphin and similarly fast and agile.

The sensory pits found on our pliosaur's snout may have acted like miniature pressure pads, detecting the turbulence produced by ichthyosaurs as they swam through deep water.

In effect, our pliosaur was able to stalk its prey even in the darkest depths just by using its skin.

♪♪ There are animals today that have similar sensory systems.

Crocodiles have over 9,000 pressure receptors, which are concentrated on their snouts, each one of which is thought to be 10 times more sensitive than a human's fingertip.

Two weeks into the dig and the work is proving to be even harder than anyone was expecting.

♪♪ Moore: Let me throw this big chunk away.

Attenborough: But at last, there's a breakthrough.

Moore: There is something under there, which is huge.

Here you've got the jaw showing and this one is heading back this way.

It's gonna be the underneath of the skull.

Etches: There's a vertebrae there; there's another vertebrae there.

Attenborough: Finding these bones confirms that the entire skull really is inside the cliff.

Moore: It's nice to actually see something.

You feel like you've been rewarded a bit.

Etches: What Alex is doing here is chipping out a loose tooth.

Hang on, that's a crown.

-Alex: Yeah.

Etches: Oh, that's interesting.

Yeah, keep going.

This is part of the root and there is a pulp cavity there.

That tooth would be probably about 10 inches long.

[ Thunder rumbling ] Moore: Oh, the thunder is coming.

Oh, look at that.

Attenborough: Just as the dig is revealing further exciting finds... -Man: Come on, guys.

-Etches: Yeah, come on.

Man: Because we've got a lot to do.

We've got to get you all up.

Attenborough: ...conditions become treacherous and the rope safety team decides that it's too dangerous to continue.

Etches: Now it's rained, this lithified mudstone has turned to like a slippery clay; it's lethal.

There's certainly no way now, with us stopping now, that we're gonna get it done in those days left.

Attenborough: The weather may be against them, but finding a pliosaur tooth is a real stroke of luck.

It could also help us understand more about the type of prey our sea monster could eat.

The teeth of marine predators vary according to their function.

Some are used to shred and slice.

Others to grasp or crush.

So what do we know about the teeth of our pliosaur?

[ Machine beeping ] Back in Southampton, the investigation of the pliosaur snout has revealed something even more intriguing.

These teeth are extraordinarily well preserved, aren't they?

Is there new information that we can get from this?

Gostling: Well, from the CT scan, if we take it back again so we can expose the teeth, these are large pointed teeth and these are really well adapted for grasping slippery fish, but this is on an order of magnitude larger, which would have allowed it to eat all sorts of prey that are swimming around in the ancient Kimmeridge Bay of the time.

Attenborough: Were the teeth permanent or were they replaced?

Gostling: As we move it back through, we can see that we've got individual teeth almost all the way along.

However, in one place, we've got this little tooth here and this is a replacement tooth.

The thing that most animals die of is that their teeth have worn out and they can't feed anymore.

And if you are a large predator, and you are catching large prey, you might lose teeth relatively frequently.

Well, this isn't such an issue if you can replace your teeth multiple times throughout your life.

Attenborough: Not only were its teeth replaceable, but they were also shaped differently -- long and sharp towards the front of its jaws, more hook-like at the back.

This deadly combination meant that pliosaurs could feed in a variety of ways, from grabbing large sharks and squid to gripping smaller, slippery fish.

♪♪ Three weeks into the dig, the weather is holding.

And now the biggest question for Steve and Chris is how to lift the skull off the cliff.

Etches: I reckon we take out a big slab.

-Moore: One piece?

-Etches: Cut it underneath.

-Moore: What, cut it underneath?

-Etches: Yeah.

-Moore: All the way through.

-Etches: Yeah.

I can't see any other way of doing it.

If we do it any other way, it's gonna just crumble up.

What do you think?

Moore: I think I'm going home and not coming back.

[ Laughs ] Attenborough: Luckily for Steve and Chris, friend and local farmer Robert Vernicom has been devising a solution.

He's building a crate which will be lowered down the cliff into which the skull will be maneuvered and then hauled up to the top.

Vernicom: The theory is that whatever angle the skids are as it comes up the cliff, the box stays level to protect the fossil because we're trying to keep the fossil as level as possible.

Attenborough: The big day finally arrives, and a local army of helpers is assembled for this crucial stage of the operation.

And Steve is feeling the pressure.

Etches: Well, the best in the world, it looks like it's all gonna function, but the risk is immense.

You know, what happens if it just actually turned on its side?

There's a lot of things that could go wrong, so it's a risky sort of time.

Attenborough: This is one of the largest and best-preserved pliosaur skulls ever found.

So the stakes are very high indeed.

[ Indistinct conversations ] ♪♪ Etches: When it comes down, we've got to get that aligned and we got to get that jaw, that skull, inside that box.

Moore: And we've got to be really careful.

That sled has got a metal bar and as it comes down, it doesn't hit the nose.

Etches: No one has ever done this before, ever.

Moore: Extracting a giant skull halfway down a cliff face?

Attenborough: The crate must be lowered into position with great precision if the skull is not to be damaged.

Moore: The position it's coming down in, it's almost -- it's gonna glance the side of the skull.

Woman: Stop!

-Man on radio: Stop!

-Woman: Stop.

Moore: So somehow we've got to manually try and move the whole sled over.

Etches: I think we can afford to do 30 centimeters lower and no more.

Attenborough: After weeks of backbreaking work, emotions are running high.

-Etches: Down!

-Moore: Quick, quicker!

This moment is really fraught.

We've got one skid just about glancing side of the skull and we've got to try and pull it out now to get it over the edge.

Vernicom: Very slowly.

6 inches.

Attenborough: One clumsy move and the skull could be smashed.

♪♪ Etches: Oh, my... Moore: Pull, pull again.

Missed it.

We've done it.

Vernicom: Stop, stop!

Etches: It's all pretty stressful.

Every part of this is really, really stressful.

Moore: After quite a few hours, we have got it into position.

We haven't knocked the end of the snout off, so far, and it's all ready to go.

Attenborough: But the next stage looks even more risky.

Man: Just retreat from the edge, and we're going up the ropes now.

Moore: Al and Steve are gonna go up to the hole and attach the winch and then start the process of slowly dragging it into the box.

Attenborough: Shifting a fossil that weighs over half a ton is really dangerous.

Moore: Move in.

Very nervous and it's very tense, so let's see how it goes.

♪♪ Etches: Ooh.

Go on.

That's better.

Man: Whoa!

Right, I think slide it straight in.

Moore: Steve, is it okay?

Has it come off okay?

Etches: Well, we're losing a bit of it, but we can't help it, mate.

Go on.

Go on.

Now it's on it.

There you go.

Alex: Good, that will do.

That's well in.

Etches: It's right in the case now.

Vernicom: Yay, the fossil's in.

Woman: Brilliant.

Brilliant.

We're all cheering here.

Well done, well done.

Vernicom: Well done, everybody.

Moore: Right.

Well done.

Brilliant.

We've overcome a lot of problems to get this far and we've done it by the skin of our teeth.

Attenborough: The skull at last is in the crate.

Etches: Take up the slack again.

Attenborough: But shifting it carelessly, let alone dropping it, could be disastrous.

Vernicom: Come over a bit.

So there will be six ropes coming up to be able to lift it and hold it steady... hopefully.

With the tide coming in and the sun setting, we could cut at any point.

Okay, right.

Whoa!

Whoa, whoa, whoa.

Attenborough: Finally, the skull is safe.

-Moore: Brilliant, Rob.

-Vernicom: Well done.

Moore: Well done.

It's amazing.

Etches: It's a dream come true and I tell you what, I don't think anyone in their right would ever believe we could have ever done it.

Three weeks ago, it was buried in a cliff face.

Found a top 13 predator and now we're bringing it back to life and this will be one of the best that's ever been found.

Moore: Good on you.

Etches: Oh, none of that.

Get off, you little devils.

God, dear, oh, dear!

It's out.

The next stage starts.

♪♪ Attenborough: The skull is transported to Steve's workshop.

Man: Ready, go.

Right up.

Moore: Oh, my God.

Etches: The mudstone has cracked and dried.

Of course what it's done is just cracked the bone as well.

Jeepers creepers.

Moore: You'll be fine.

Etches: Oh, yeah.

Attenborough: The painstaking task of removing the stone from around the skull so that it can be examined in detail... ...can at last begin.

After the heavy work of the dig, this stage requires a delicate touch.

First, Steve removes the rocks around in the fossil so that the fragile areas of the skull can be strengthened.

Then, using an air-abrasion tool, he starts work to reveal the more intricate details about the anatomy of this extraordinary animal.

This giant sea monster, after 150 million years, finally begins to emerge from the rock.

And I have the privilege of coming to see this whole skull for the first time.

So here it is.

And it's enormous.

I am meeting Dr. Judyth Sassoon, a paleontologist who has studied pliosaur specimens for decades.

Does it still take your breath away as it takes mine?

Sassoon: It is the most astonishing specimen, David.

I'm very pleased to be part of the work on it.

Attenborough: What insights can Judyth give us into the life of this ancient monster?

How's it coming along?

What sort of detail you can get from this, which you never knew before?

Sassoon: Steve Etches has been working on it now for several months and has made some fabulous progress.

We're seeing, as it's being prepared, gradually more and more detail being revealed.

So far we have some information about its senses.

Attenborough: Really?

Was their eyesight good?

Sassoon: There are indications that, in fact, it could have been.

The eyes themselves were quite important for this animal.

One of the reasons is the position itself.

The eyes are on the side of the head, more or less in the middle, so not too high and not too low.

Attenborough: This important feature of the skull may suggest how our pliosaur hunted in the open ocean.

The position of the eyes in living animals varies according to the way in which they hunt.

Dolphins are pursuit hunters.

Their eyes are placed on the side of their heads giving them panoramic vision... ...helping them to attack their prey more accurately.

Ambush predators, such as crocodiles, have eyes higher up on their heads.

So they can remain just below the surface with only their eyes above water and judge when to attack.

♪♪ Our pliosaur seems to have had something in between... ...with an eye position that not only enabled it to pursue prey through water with accuracy but alternatively surprise it by attacking from below.

And there is another remarkable feature, which could tell us more about where our sea monster may have hunted.

Sassoon: We talked about eyes, there is also another interesting structure, which is the parietal eye.

In many reptiles, this still exists.

When it is present in terrestrial animals, it has a full eye structure like the lateral eyes and is light-sensitive.

Attenborough: The parietal eye on the top of the head is something of a puzzle.

It's known as a third eye and is still found in a few living species.

It apparently helps an animal to regulate its body clock.

♪♪ The pliosaur's parietal eye is thought to have had a lens, a cornea and a retina.

Although its exact function is unclear, it may have enabled our sea monster to gauge which way was up when swimming at depth and potentially navigate deeper hunting grounds.

What other questions would you have wished the skull to provide answers for?

Sassoon: I've already made some measurements on this animal and the proportions do seem to be different from other pliosaurs that we know.

The skull is quite long-snouted, but the position of the nose and the eye and also of the crest suggest that it is something else, something new.

Attenborough: A new species of pliosaur?

Sassoon: Of pliosaur.

-Attenborough: A new species?

-Sassoon: Yes.

Attenborough: Really?

Sassoon: I think it could be.

Yes.

Attenborough: The revelation that our pliosaur could be a new species is truly exciting.

♪♪ There are only eight recognized species of pliosaur and this skull is certain to provide new scientific data on the evolution of these mysterious marine reptiles.

I have to say, you take my breath away.

The detail which you can deduce, it is mind-blowing, I think.

I mean, that's what paleontology is about.

I used to think it was just a question of finding a fossil and digging it out and saying how nice it was.

You've made it sound rather different.

♪♪ In recent decades, huge advances have been made in our ability to study prehistoric animals.

And we can now investigate the predator power of our pliosaur in more detail than ever before.

Paleobiologist Dr. Andre Rowe is a world-leading expert in 3D visualization of fossils.

Rowe: The first thoughts -- this thing is absolutely massive.

And I will also add that the level of preservation is amazing.

This is actually a one in a million, maybe one in a billion type specimen here.

Attenborough: Using the latest technology, Andre is carrying out the world's first surface scan of a pliosaur skull.

Rowe: So right now, we're capturing basically hundreds of thousands of images all at once.

The end result is a really nice-looking 3D model.

I think we'll be able to unlock a lot of mysteries about what these sea monsters were doing and I'm really excited to see where it takes us.

Attenborough: Once the scan has been finalized, I meet Andre at the University of Bristol to discuss his findings.

Has he seen anything at the skull structure that shows our sea monster had the power of a truly deadly predator?

Rowe: There are some massive openings back here, along the jaw line, and that's good for muscles to attach and bulge out.

Attenborough: There would have been a muscle running through there.

Rowe: Yes, we have the pterygoid muscle group, which is in a lot of big dinosaurs.

That's integral to having a really strong bite.

We've hypothesized that this particular pliosaur is kind of the apex predator in the Jurassic ecosystems it was living in.

Attenborough: Now, this is, of course, not a dinosaur, but neither is it a modern reptile, but it does look a lot like a crocodile, doesn't it?

Rowe: Yes, well, we have this process called convergent evolution, where a lot of animals will adapt a similar-shaped skull or similar trends throughout their evolutionary history even if they're not closely related.

That's because having those similarities can give you a lot of advantages, but in terms of this pliosaur, it's got that kind of streamlined skull that's kind of broad on the back.

It's very triangular-shaped.

I think that it's good for swimming quickly and ambushing prey.

Attenborough: Does the skull give you any information about what animal of prey might have been?

Rowe: The animal would have been so massive that I think it would have been able to prey effectively on anything that was unfortunate enough to be in its space.

A popular hypothesis is that these animals were actually ripping off the limbs of other animals and disabled them from swimming away and then kind of going in for the kill.

Attenborough: So this is a top predator?

Rowe: Yes, I have very little doubt just judging from how massive that skull is.

I don't see what could have possibly hurt it.

Attenborough: What size is that actually?

Rowe: So the actual skull itself comes in at about 6.5 feet, about 6'6".

Attenborough: So it's longer than I am tall?

Rowe: Yes, it's quite a big boy and that's just the skull.

Attenborough: The dimensions of a fossilized skull enable us to estimate the overall size of an animal when it was alive.

And based on Andre's measurements, our pliosaur could have been up to an astounding 40 feet long.

Rowe: Just on the sheer size of it, just from looking at this animal and how big those pterygoid muscles would have been at the back of the jaw, the animal would have delivered a devastating bite, no doubt.

Attenborough: A powerful bite is vital to the success of any marine predator.

Scientists are able to estimate how much force an animal can exert when biting into its prey.

And great white sharks have one of the strongest bites... at around 10,000 newtons.

But how do you work out the bite force of a creature that became extinct millions of years ago?

Professor Emily Rayfield is a world-renowned paleontologist who specializes in skeletal mechanics.

Using the model created by Andre, Emily has assessed the bite force of our pliosaur.

Rayfield: So this is a 3D print, a model, it's not full size, though it's just over a third of the size of the actual animal.

These large openings are the spaces in the skull, which would have been filled with jaw-closing muscles.

Attenborough: So you can estimate the force of the bite from the size of those muscles?

-Rayfield: Exactly, yeah.

We can get an estimate of that from here.

We know that muscles of a certain -- a certain size, a certain area, are capable of generating a certain amount of force.

Saltwater crocodiles have got the largest-ever bite force measured, and they're up to about 16,000 newtons.

Attenborough: And these?

Rayfield: So our pliosaur here, from the estimations that we've made, has a bit force that's about twice the size of that -- the larger saltwater crocodile that's ever been measured.

And it's in the region of around 32,000 newtons.

Attenborough: So this is the most powerful biter in the sea that ever has been or that we know of.

Rayfield: That we know of, absolutely, yes, definitely.

If you're looking at kind of statistics in terms of car-biting metrics, I'm pretty sure it could probably bite through a car.

-Attenborough: So it's a monster.

Rayfield: Absolutely.

[ Both laughing ] ♪♪ Attenborough: The evidence gathered from the skull so far suggests that this pliosaur had the jaws, teeth and senses of a highly successful hunter.

Its long stout, short neck and streamlined skull enable it to move easily through the water.

But what else helped our enormous sea monster to power through the Jurassic seas fast enough to catch its prey?

Pliosaurs were unique in the natural world as they had four almost identical wing-like flippers.

How pliosaurs used their flippers has been debated by paleontologists for decades.

Some believe that they moved using a sort of rowing stroke, like oars in a boat.

While others argued they used a flight stroke, seen in animals such as sea turtles.

But in recent years, scientists have been able to use computer modelling to finally solve this mystery.

And it appears that these giant sea monsters swam in a way that is surprisingly similar to a very different type of animal, one that is alive today.

♪♪ Whoa.

Penguins may appear somewhat clumsy as they waddle around on land.

But once they're in the water, they move very differently.

These are Humboldt penguins and they're excellent swimmers.

Their streamlined body shape and their oily feathers enable them to reach astonishing speeds of up to 30 miles an hour.

But a key factor behind penguins' speed are their flippers, which underwater, act like propellers driving them forward and increasing their speed dramatically.

In slow motion, you can see that the penguins are using a lift-based underwater flight movement, twisting their wings as they flap and propelling themselves forward on the upstroke as well as the downstroke.

As strange as it may seem, it's thought that pliosaurs would have moved in a very similar way.

But, of course, pliosaurs were enormous and most large animals are relatively slow-moving.

So as an apex predator, how could this huge creature maneuver itself fast enough to catch its prey?

To find out, I've come to the hydrodynamic laboratory at Imperial College London where Dr. Luke Muscutt is studying the locomotion of pliosaurs using a rather unusual research tool.

How did you first become interested in the way that pliosaurs swam?

Muscutt: The only animal that we know of that has four large flippers.

So the question is, how did they use them?

The fossils of the pliosaur show that the flippers were very much like wings.

So what I found was that the hind flipper can actually operate at a much higher thrust and a much higher efficiency because it's utilizing the wake of the flipper in front of it.

We can see a similar effect in the flight of migrating birds, such as geese.

Attenborough: When geese are flying in formation, each bird benefits from the uplift created by the one in front of it so that they fly in a very energy-efficient way.

Muscutt: So you can think of the pliosaur as almost two birds, one flying behind the other, and the back one is benefiting from the one in front.

Attenborough: That's an extraordinary parallel, yes.

Muscutt: The hind flipper has increases in thrust and efficiency of up to 40%.

-Attenborough: Ahh.

Muscutt: So this would have increased the swimming speed that pliosaurs would have been able to achieve and increase the number of different things it could eat.

Attenborough: To take his research to the next level, Luke has built a robot to study the swimming pattern of pliosaurs more accurately than ever before.

So what more information do you think you can get from this model?

Muscutt: This robot enables me to test the complete animal.

How fast something can move is an absolutely critical part of what that animal is and it tells us what animals it could have eaten, how far it might have been able to travel, all sorts of questions come back down to its locomotion ability.

Attenborough: Have you estimated a speed that this might produce?

Muscutt: Well, I've only finished building this yesterday so... -Attenborough: Oh, really?

Muscutt: So far, I haven't actually ran the experiments yet.

If you would like to have a go, you're more than welcome to... Attenborough: Yes.

Oh, show me.

Muscutt: So if you just move this joystick sort of upwards further...

So this is how the pliosaur would have swam.

The flippers move primarily up and down.

It's much more like a bird flies.

Attenborough: Luke and his team set up the robot for a test swim, and they entrust me with its maiden voyage.

Muscutt: If you would like to take the control...

There we are.

It's off.

Attenborough: I suppose -- actually that's only a model -- but if it was full size, it would be going quite fast.

Muscutt: Indeed.

You can just imagine it chasing after a smaller ichthyosaur.

Attenborough: Luke's research is so new, it's yet to be published.

But it's helping to provide a new perspective on these extraordinary animals.

♪♪ Large marine predators like minke whales and orcas can swim at great speed through the ocean.

What speed might our pliosaur, with its four flippers, have been capable of?

Estimates suggest that they could have accelerated up to 30 miles an hour, making them one of the fastest animals in the Jurassic seas.

♪♪ This skull is not only helping us to understand more about the lives of these giant sea monsters but also allows scientists like Dr. Andre Rowe to visualize the Jurassic world as never before.

So often, I've been involved in looking at fossil studies and the skull, unless the skull is there, you are really missing an awful lot of information.

We are lucky to find this as a first thing.

Rowe: Yes.

I'm very biased since I study feeding and teeth, but I think the majority of information about an animal, you can get from its skull.

The brain, the teeth, what it was feeding on, its maximum body size, if you have the whole skull.

It's just a treasure trove of information and we're very fortunate to have the whole thing.

One of the reasons why I love the UK is because it's got such a great collection of marine reptiles.

Attenborough: [ Laughs ] Delighted to hear it.

Rowe: I mean, in America, we've got our big tyrannosaurus and our triceratops, but UK is great for marine reptiles.

Attenborough: But we did discover the dinosaurs.

Rowe: Yes, the science of paleontology did originate here.

Attenborough: How would it compare with T-rex?

Rowe: I imagine it would be pretty comparable and they were kind of both the respective apex predators in their ecosystems.

So I have no doubt that this is sort of like an underwater T-rex if you will.

Attenborough: Okay, let me ask you the million-dollar question.

In a battle between T-rex and our pliosaur, who's gonna win?

Rowe: As much as it pains me and brings a tear to my eye to admit it, I think my T-rex is going to lose this fight.

Millions of years later, an American paleontologist will envision the scene and break down into tears.

Attenborough: [ Laughs ] Bringing an enormous predator back to life after 150 million years is no easy task.

But restoring this giant skull is a labor of love for Steve and his team.

Almost a year after the skull was discovered, I return to Kimmeridge to see how they're getting on.

My goodness.

It is absolutely magnificent.

It's astonishing.

Moore: It's bigger than a T-rex.

-Attenborough: Is it?

-Moore: Yeah, yeah.

-Attenborough: What, the skull?

-Moore: Yup.

Bigger than any T-rex ever found.

Etches: David, now what we've done, since you've come here last, is we've -- I've air-penned off all the mudstone and then air-brated it.

Now, the air-abrasive machine cleans out all these little voids and you see every little detail, every suture where the bone joined together, you can see every detail, that's what we really wanted.

Moore: So the teeth here have been basically tumbled on the beach and the shingle has worn away all the crowns so we can do a bit of dentistry on them.

So we've got this tooth, which has been scanned, and then we're gonna increase or decrease it, and then add all the teeth back in position.

Show people what it actually looked like.

Attenborough: You must feel, looking at this -- I mean, I know it was a huge amount of work to get it out.

Etches: We never thought we'd get it, to be honest.

I'll be honest with you.

Attenborough: Well, it's certainly a triumph.

Etches: Yeah, quite an emotional moment for everyone.

Attenborough: Yeah, I'm sure.

It's sensational.

♪♪ Once the pliosaur's dagger-like teeth are added, the picture is finally complete.

♪♪ Our journey of discovery has shown that this sea monster was one of the greatest predators the world has ever seen.

And we can now visualize, more accurately than ever, how it may have hunted in the Jurassic seas.

♪♪ Ichthyosaurs... swimming in groups along the coast... concentrating on hunting their prey, unaware that they themselves are being stalked.

On their trail, our pliosaur uses its highly tuned senses to launch an attack.

♪♪ In the chase, its four flippers, each measuring over 6 feet long, drive it through the water at great speed.

♪♪ Splitting the shoal, it isolates its target.

Our sea monster's primary weapons are its 90 razor-sharp teeth... ...with which it slices through its victims' flesh.

The impact alone may have been enough to kill.

But with a bite force twice the strength of any animal living today, its prey had little chance of survival.

♪♪ ♪♪ ♪♪ From a chance discovery on a beach one morning to the painstaking restoration of such a rare and impressive specimen, the story of this fossil is one of skill, dedication and of fascinating scientific discoveries made along the way.

We've been given a unique insight into the life of our pliosaur that swam in the Jurassic seas 150 million years ago... but we're also reminded that there is still so much to learn about these extraordinary prehistoric animals.

And I, for one, will never tire of discovering more.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ To learn more about what you've seen on this "Nature" program, visit pbs.org.

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